1. Technical Field
The present invention generally relates to a developing device to develop a latent image with developer and an image forming apparatus, such as copier, a printer, a facsimile machine, a plotter, or a multifunction peripheral (MFP) having at least two of coping, printing, facsimile transmission, plotting, and scanning capabilities, that includes the same.
2. Description of the Background Art
In image forming apparatuses employing a developing device, typically an exposure device forms a electrostatic latent image on an image bearer, such as a photoreceptor, according to image data, and the developing device develops the latent image into a toner image. The toner image is then transferred onto a recording medium either directly or via an intermediate transfer member (i.e., an intermediate transfer belt) and fixed thereon by a fixing device.
There are developing devices that use two-component developer consisting essentially of toner and magnetic carrier (inclusive of external additives) and one-component developer consisting essentially of toner (inclusive of external additives).
For example, two-component type developing devices agitate developer inside the developing device to mix together toner and magnetic carrier, adjust the concentration of toner in two-component developer (hereinafter simply “developer”) to a proper value, and then supply the developer to a developer bearer such as a developing roller. The amount of developer carried on the developing roller is adjusted by a developer regulator such as a doctor blade, after which the developer is transported to a development range where the developing roller faces the photoreceptor through an opening formed in a development casing. Then, developer (i.e., toner therein) adheres to the electrostatic latent image formed on the photoreceptor, thereby developing it. Most of toner particles inside the developing device is adsorbed onto the surfaces of carries through frictional charging with carriers and caused to adhere to the electrostatic latent image on the photoreceptor due to the effects of developing electrical fields generated between the developing roller and the photoreceptor in the development range.
Powdered toner, however, includes carrier particles whose charging capabilities are insufficient and toner particles insufficiently charged by friction with carrier particles. Further, the charging capabilities of carrier in the developing device may deteriorate over time. Such insufficiently charged toner escapes electric restraint and floats inside the device, entrained on airflow generated inside the device. When the average of pressure inside the entire developing device (hereinafter simply “internal pressure”) is high, it is possible that floating toner spurts out the developing device through clearance and scatters.
Internal pressure rises inside the developing device, which invite toner scattering, are described below.
Internal pressure rises are caused by changes in airflow due to rotation of a transport roller or the developing roller. In particular, the developing roller is exposed to the external air, and an exit side (at an upstream rim of the opening of the development casing) in the direction of rotation thereof is closed by the doctor blade and developer. On an entry side (at a downstream rim of the opening) in the direction of rotation of the developing roller, clearance called a casing gap (CG) is present between the developing roller and the downstream rim of the opening, and sucking-in airflow is generated as the developing roller rotates, thereby increasing the internal pressure. The casing gap (CG) is an important factor in development.
Although the internal pressure is raised by sucking-in airflow caused by rotation of the developing roller, air suction is not uniform in the longitudinal direction of the developing roller. Although it depends on differences in the casing gap CG, the deference between the end portions and the center portion is caused by side plates of the development casing. Since the end portions are closer to the respective side plates, the internal pressure rises therein. Specifically, pressure does not rise in the center portion since the sucked in air flows to the left and right. In the end portions, however, the airflow is blocked by the side plates, and accordingly pressure is likely to rise.
Additionally, a magnet provided inside the developing roller is shorter than the developing roller, and the end portions of the developing roller are outside the magnetic force range. Accordingly, developer is not transported in the end portions, and airflow arising inside the magnetic force range surges to the end portions where airflow is not generated. Further, beneath the developing roller, a developer conveyance member such as a conveying screw circulates developer laterally and influences the airflow. It is deemed that, due to this influence, development quality in the both end portions is not identical, and the scattering of toner from one side is greater.
JP-2009-223075-A (JP-5106191-B) and JP-2010-237635-A mentions that, as the developing roller rotates, air flows in through the casing gap, and pressure inside the device rises, fostering toner scattering therefrom. JP-2009-223075-A proposes a configuration in which a suction port is formed in an inner wall of the development casing and extends long in the longitudinal direction of the development casing, a filter is provided to the suction port, and an exhaust space is provided in the longitudinal direction of the development casing covered with a filter cover. In this configuration, airflow inside the development casing is guided to vents (i.e., openings) formed in longitudinal end portions of the developing device and discharged outside the image forming apparatus to suppress internal pressure rises.
By contrast, JP-2010-237635-A proposes forming a suction port in an inclined wall of the development casing on a side opposite the image bearer to guide the airflow to the outside of the image forming apparatus. The suction port is constructed of multiple through holes identical in shape and arranged symmetrically in the lateral direction, at regular intervals in the longitudinal direction of the development casing. Then, air is discharged outside the apparatus via a filter from the development casing through the multiple through holes dispersed in the longitudinal direction to suppress internal pressure rises.
Additionally, JP-2010-008978-A proposes a configuration to discharge airflow inside a developer chamber upward from an upward opening positioned in an upper part of the development casing. JP-2010-054932-A proposes a configuration in which airflow at a communicating opening in an upper part of the development casing is discharged upward from an upward slit.
Further, JP-2007-140288-A and JP-2009-020276-A propose discharging airflow from the developer casing upward through a pressure-release opening positioned in an upper part of the development casing. Yet further, JP-2010-217425-A proposes discharging airflow from the developer casing obliquely upward through a pressure-release opening positioned in an inclined wall of an upper part of the development casing.
It is to be noted that JP-2006-145921 (JP-4535852-B) mentions a configuration in which, airflow at the casing gap goes inside the development casing in a center portion in the width direction of the casing gap and goes outside at both ends in that direction. In this configuration, an end portion in the width direction of an upper rim (a face opposed to the developing roller) at the casing gap is made longer in length in the direction of rotation of the developer bearer than in a center portion in the width direction to inhibit airflow from going outside, thereby inhibiting toner scattering.